1. Field of the Invention
The present invention relates to a process for preparing 5-aminolevulinic acid or a salt thereof which is useful as a raw material for various chemicals and is an agricultural chemical. The process can be applied to the industrial manufacture of 5-aminolevulinic acid with advantage.
2. Description of the Background Art
5-Aminolevulinic acid is an intermediate for biosynthesis of haem, vitamin B.sub.12, and cytochrome. It is known to be a useful compound as a raw material for various chemicals and also as an agricultural chemical. This compound has been reported to exhibit a selective herbicidal action by Revitz et al. at Illinois University.
The 5-aminolevulinic acid can be prepared either by using photosynthetic microorganisms or by chemical synthetic methods. Included in typical known methods of chemical synthesis of 5-aminolevulinic acid are, (1) a method of converting succinic acid into a monoester and introducing a carbon-nitrogen unit into the monoester, followed by reduction; (2) a method of oxidizing levulinic acid to protect the 2- and 3-positions as a double bond, aminating the 5-position carbon atom of the protected compound, and reducing the amino compound; and (3) a method of oxidizing 2-hydroxypyridine to 2,5-dihydroxypyridine, reducing the 2,5-dihydroxypyridine into 2,5-pyperidinedione, and hydrolyzing the 2,5-pyperidinedione.
Problems encountered in the above-mentioned method (1) are the necessity of protecting one carboxyl group of succinic acid, the requirement of using a deadly toxic cyano compound, such as silver cyanide or copper cyanide, for introducing the carbon-nitrogen unit, and also the need for handling toxic compounds such as zinc and the like during the reduction. In the absence of a method of selectively introducing an amino group to a desired position, the method (2) requires the complicated procedure of protecting and removing the protective groups from the positions which otherwise need not be modified. Notwithstanding the comparatively short processing steps involved, the method (3) has drawbacks in the high cost of the raw material 2-hydroxypyridine, the difficulty involved in the availability of this raw material, and the low overall yield of as low as about 8-10%.
More recently, comparatively reasonable methods for synthesizing 5-aminolevulinic acid have been disclosed. One comprises providing N-protected furfurylamine (furfurylamine of which the amino group has been protected), oxidating this compound by electrode or bromine in the presence of methanol, and reducing the oxidated compound, followed by further oxidation with KMnO.sub.4. In another method, tetrahydrofurfurylamine obtained by the reduction of furfurylamine is used as a raw material for oxidation with ruthenium oxide.
One problem with these methods is the use of a large quantity of expensive chemical oxidizing agents, which gives rise to a high production cost. In particular, KMnO.sub.4 used in the former method cannot be recovered in the form usable as an oxidizer, and therefore must be discarded. Difficulties are encountered in the waste liquid treatment. In addition, because only an insufficient amount of oxidation is achieved in the first oxidation step, the oxidation must be carried out in two stages.